The extracellular signal-regulated kinase (ERK)/MAP kinase pathway is a major control point for mesenchymal differentiation. In bone, this pathway is a major conduit for conveying information about the extracellular environment to the nucleus and has been implicated in the response of bone to a variety of signals including cell adhesion, hormone/growth factor stimulation and mechanical loading. As shown by this laboratory, ERK/MAPK signaling is necessary for osteoblast differentiation and in vivo skeletal development. Furthermore, actions of this pathway on osteoblast differentiation are mediated by phosphorylation and activation of the bone-related transcription factor, RUNX2, at 2 critical serine residues. Goals for this renewal are to understand how phosphorylation controls RUNX2 transcriptional activity and establish the physiological significance of MAPK signaling and RUNX2 phosphorylation during development and post-natal life. Aims are: 1. Establish the importance of MAPK phosphorylation to the biological activity of RUNX2 in vitro and the underlying mechanism of transcriptional activation. 2. Evaluate the role of the ERK/MAPK pathway in osteoblast/osteocyte function in adult mice. 3. Evalulate the in vivo role of RUNX2 phosphorylation in skeletal development and homeostasis. These studies will establish the significance of a novel pathway for controlling gene expression in bone. Mechanisms defined in this project have the potential to explain how bone responds to a number of primary extracellular stimuli including ECM and mechanical signals and may serve as the basis for new therapeutic approaches targeting MAPK signaling.7.